This invention relates generally to implantable stimulators and, more specifically, to implantable pacemakers, cardioverters and defibrillators.
Over the years, numerous methods have been proposed for pacing the heart in an attempt to interrupt tachycardias. These include such pacing modalities as overdrive pacing, burst pacing, autodecremental overdrive pacing, and others. These pacing modalities have been formulated to interrupt conduction on aberrant cardiac conduction paths, which may lead to sustained tachycardias in one or more chambers of the heart.
In recent years, attention has been focused on the use of pacing pulses delivered using standard cardiac pacing electrode systems located in the atrium and/or ventricle. Typical electrode systems for delivery of antitachycardia pacing pulses have included: unipolar systems, utilizing an electrode located on or in one chamber of the heart and a remote electrode; bipolar systems, employing two electrodes located on or in a chamber of the heart; and integrated bipolar systems, employing a small surface area pacing electrode located on or in a chamber of the heart in conjunction with a large surface area electrode located on the heart, typically a defibrillation electrode. Each of these approaches to pacing assumes that the active pacing electrode is a single small surface area electrode, and that propagation of the depolarization wavefront within the heart begins adjacent to the pacing electrode, and then propagates throughout the heart from the tissue adjacent the pacing electrode.
A commonly held principle of antitachycardia pacing is that paced termination of a tachycardia is facilitated if the stimulating electrode is positioned close to the reentrant circuit. This condition is often difficult to achieve since electrodes cannot be readily placed in many regions of the heart, nor is it always evident where the preferred placement lies.
In the past, it was proposed that tachycardias could be interrupted by the use of multi-site cardiac pacing. One early example of multi-site cardiac pacing to terminate or prevent tachyarrhythmia is disclosed in U.S. Pat. No. 3,937,226 issued to Funke. In this device, a number of small surface area pacing electrodes are provided, each coupled to a separate output circuit and amplifier. The disclosed device is equivalent to five or more separate cardiac pacemaker output circuits of conventional design, all adapted to be triggered to pace simultaneously at various locations around the heart. It is hypothesized that by stimulating simultaneously at locations spread around the heart, synchronous with a sensed QRS complex, arrhythmias could be prevented by producing a more nearly simultaneous depolarization of cardiac tissues.